The present invention relates to an alumina ceramic composition, more particularly to an alumina ceramic composition exhibiting a high Q factor in a high frequency band such as a radio frequency band, a microwave band, etc.
For circuit substrates used in a high-frequency band and parts for microwave transmission apparatuses, alumina ceramic compositions having small dielectric loss are required to reduce transmission loss. Though so-called high-purity alumina having a purity of about 99.999% is a material having a small dielectric loss, it cannot advantageously be used for alumina ceramic compositions for parts of the above microwave transmission apparatuses, etc., because its sintering temperature is disadvantageously as high as 1600xc2x0 C. For instance, when the sintering temperature exceeds 1500xc2x0 C., higher-quality sintering furnaces are needed, resulting in decrease in productivity and higher production cost. Also, the high-purity alumina per se is so expensive that it cannot be used for parts of microwave transmission apparatuses, etc., which are required to be small and inexpensive with high performance.
Thus widely used as inexpensive alumina materials having a purity of about 99.8% for parts of microwave transmission apparatuses, etc. are alumina ceramic compositions with improved sinterability by lowering a sintering temperature to 1400 1500xc2x0 C. with small amounts of MgO or TiO2. Though the addition of MgO, etc. is effective to lower the sintering temperature, it tends to increase the dielectric constant of the resultant alumina ceramic compositions. As a result, such alumina ceramic compositions as containing MgO or TiO2 fail to have sufficient Q factors for practical applications due to extremely increased dielectric loss as compared with high-purity alumina.
Disclosed as an alumina ceramic composition with lowered dielectric loss and increased Q factor in Japanese Patent Laid-Open No. 4-356922 is an alumina ceramic composition having a dielectric loss (tan xcex4) of 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x923 and a Q factor of 1,000 to 10,000 by limiting the total content of alkali metals (as Na2O, K2O) to 150 ppm or less. Further, an alumina ceramic composition disclosed in Japanese Patent Laid-Open No. 8-59338 has a Q factor of 10,000 or more by limiting the total content of alkali metals (calculated as Na2O, K2O) to 100 ppm or less and the content of Mg (calculated as MgO) to a particular range of 100 ppm or less or 1,000 to 50,000 ppm. Japanese Patent Laid-Open No. 8-59338 suggests that inevitable impurities such as Si, Fe, Ca, Ti, etc. are contained in the alumina ceramic composition, and that the amount of each impurity is 500 ppm or less.
However, both of Japanese Patent Laid-Open Nos. 4-356922 and 8-59338 are directed to alumina ceramic compositions with relatively high Q factors, in which the total content of alkali metals (as Na2O, K2O) are necessary to be limited to extremely small amounts such as 100-150 ppm or lower. For this purpose, expensive high-purity alumina is used as a starting material, or when a common-grade alumina is used as a starting material, an additional step of fully removing alkali metals is needed, resulting in higher production cost.
It has conventionally been considered that in alumina ceramic compositions, the amounts of impurities dissolved in grain boundaries and alumina crystal grains contribute to increase in their dielectric loss. Generally adopted to obtain alumina ceramic compositions with low dielectric loss is to use as a starting material high-purity alumina, in which the total content of impurities such as alkali metals (Na2O, K2O) are limited. As described above, however, this may deteriorate the productivity and sinterability of alumina ceramic compositions. Thus, there has conventionally not been proposed an alumina ceramic composition formed from a usual low-purity alumina starting material and having low loss (high Q factor) and low-temperature sinterability.
Accordingly, an object of the present invention is to provide an alumina ceramic composition having a low dielectric loss (high Q factor) and improved sinterability and productivity, which is formed from a usual low-purity alumina starting material without using high-purity alumina.
As a result of research on the influence of various impurities and additives on the dielectric loss of an alumina ceramic composition, the inventor has found that when the amount of Mg added to improve the sinterability of alumina is within a particular range, the resultant alumina ceramic composition has an extremely decreased Q factor. Specifically, when the content of Mg is 0.2 weight % or less when calculated as MgO, the Q factor of the alumina ceramic composition is small, and when the Mg content is near 0.05 weight %, the Q factor of the alumina ceramic composition is extremely small. On the other hand, the addition of Ti in an amount of 0.2-10 weight % (calculated as TiO2) improves the Q factor of an alumina ceramic composition, which would be extremely low if otherwise, even with the Ti content of 0.2 weight % or less, for instance, 0.05 weight % (calculated as TiO2). In sum, while the dielectric loss of an alumina ceramic composition is suppressed to provide an improved Q factor by limiting the contents of alkali metals and MgO to particular ranges in the above prior art, the present invention has achieved to provide an alumina ceramic composition formed from a low-purity alumina starting material with an improved Q factor by adding both Mg and Ti in particular amounts.
Thus, the first alumina ceramic composition according to the present invention comprises 10 weight % or less (calculated as MgO) of Mg, and 0.2-10 weight % (calculated as TiO2) of Ti, the balance being substantially an Al2O3 phase and inevitable impurities.
In the first alumina ceramic composition, the content of Mg is preferably 0.2 weight % or less, more preferably about 0.05 weight %, when calculated as MgO. Also, the content of Ti added together with Mg is preferably 0.5-10 weight %, more preferably 0.75-5 weight %, when calculated as TiO2. The proportion of Mg to Ti is properly balanced within the above ranges. It is preferable that the structure of the alumina ceramic composition partially contains an Al2TiO5 phase.
The second alumina ceramic composition according to the present invention has a structure based on an Al2O3 main phase and partially containing an Al2TiO5 phase, a ratio X of [I(AT)/I(Al)xc3x97100%] being 0.2-15%, wherein I (Al) represents a diffraction intensity in a (113) plane in the Al2O3 phase, and I (AT) represents a diffraction intensity in a (110) plane in the Al2TiO5 phase, both determined from an X-ray diffraction pattern.
In the first and second alumina ceramic compositions, the Al2TiO5 phase is preferably precipitated in grain boundaries. The Al2TiO5 phase precipitated in grain boundaries preferably surrounds part of grain boundaries composed of inevitable impurities.
The third alumina ceramic composition according to the present invention is based on Al2O3 and containing Mg, Ti and inevitable impurities, wherein the value xcex94La obtained by subtracting the standard value Ls of an a-axis lattice constant of Al2O3 in the standard data list of powder X-ray diffraction patterns (JCPDS card No. 46-1212) from an a-axis lattice constant La of the Al2O3 phase in the alumina ceramic composition is in the range of xe2x88x920.003xe2x89xa6xcex94La less than 0 (unit: angstrom).
The first to third alumina ceramic compositions preferably have a Q factor of 850 or more, particularly 1000 or more, when measured at a frequency of 10 GHz. The upper limit of the Q factor is preferably about 3800. An available range of the Q factor is thus about 850 to about 3800.